If I recall correctly from my school days, (forestry major) I believe another reason for using white birch in toothpicks is that it tends to not splinter compared to other species.
Welcome to the Straight Dope Message Boards, ewood, glad to have you with us. It’s helpful, when you start a thread, if you provide a link to the Column you are discussing. Helps keep us all on the same page. Yes, it’s obvious now, but in a few days, this column will sink from the Home Page back into the archives and be harder to track.
In this case, the column is "Are toothpicks made of real wood?’